LED is rapidly replacing incandescent bulbs, fluorescent lamps, and other types of light sources due to its high efficiency, small size, high reliability, and long lifetime. FIG. 1 is a typical application of an LED used in a buck converter. As shown in FIG. 1, when a switch S1 is turned on, a switch S2 is turned off, an input VIN, an inductor L, the LED, and the switch S1 form a current loop. The current flowing through the inductor L and the LED increases. When the switch S1 is turned off, the switch S2 is turned on, the inductor L, the LED, and the switch S2 form a current loop. The current flowing through the inductor L and the LED decreases. The switch S2 is usually replaced by a freewheeling diode in use. The switch S1 is put in the low side as shown, so that no floating drive circuit is needed.
The brightness of the LED is determined by the average current that flows. As a result, accurately controlling the average current of the LED is important. There are two current control methods which are adopted by conventional buck type LED circuits. Method 1 senses the current flowing through the low-side switch. This current sensing could be realized by the switch's own conductive resistance. Then the current is regulated by peak current mode control. This current control method is simple, with no external circuit or pin needed. In the peak current mode control, the peak value of the current is accurately controlled. However, because of the influence caused by the ripple, the error of the average current is large, which causes low precision.
Method 2 adopts a current sense resistor coupled in series with the LED. The current flowing through the LED is detected by the current sense resistor. Then the current is regulated by the average current mode control. This current control method has high precision. However, the series coupled current sense resistor introduces additional power loss.